Determination of Pka - The purpose of this experiment is to determine the pKa of a series of para-substituted PDF

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Summary

The purpose of this experiment is to determine the pKa of a series of para-substituted benzoic acids to gain an understanding of how the substituents affect the ionization of the acids.
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Description

Lab Report: Determination of pKa

Purpose: The purpose of this experiment is to determine the pKa of a series of para  - substituted benzoic acids to gain an understanding of how the substituents affect the ionization of the acids.

Chemical Structures/Reagents:

Calculations:

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First Derivative Graph - slope ∆pH/ ∆V Second Derivative Graph - slope ∆( ∆pH/ ∆V)

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Slow run: Added 10 drops of NaOH over 0.6 mL 10 drops / 0.6 mL = 16.17 drops/mL 1/16.67 drops/mL = 0.06 mL drop

Results: - Nicotinic Acid was assigned to our group - 21mg of Nicotinic Acid was weighed out for use in the quick run Quick Run: Titration Data: Burette Reading

Volume NaOH (mL)

pH

0

0

4.01

0.5

0.5

4.16

0.5

1

4.25

0.5

1.5

4.32

0.5

2

4.39

0.5

2.5

4.49

0.5

3

4.56

1

4

4.66

1

5

4.77

1

6

4.93

1

7

5.08

1

8

5.24

1

9

5.41

1

10

5.65

0.5

10.5

5.84

0.5

11

6.14

0.5

11.5

6.60

0.5

12

9.47

0.5

12.5

10.61

0.5

13

11.02

0.5

13.5

11.23

0.5

14

11.35

1

15

11.56

1

16

11.60

1

17

11.68

Titration Curve:

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Estimated pKa: 5.2

Titration Data Slow Run: -

20.6 mg Nicotinic Acid were used in the slow run

Volume NaOH, Drops

pH

0

7.24

1

7.88

2

8.13

3

8.26

4

8.40

5

8.63

6

8.97

7

9.30

8

9.57

9

9.90

10

10.1

Titration Curve for Slow run

* we did not have enough data to get an accurate titration curve so that is why the curve looks skewed

First Derivative Graph

Second Derivative Graph

Discussion Questions:

1. The quick run and the slow run both yielded different equivalence points. For the slow run, our equivalence point was approximately 7.56 mL. For the fast run, our equivalence point was approximately 9.5mL. During our fast run, we added NaOH at 0.5 mL intervals and with our slow run, we added NaOH drop by drop. Based on these results, we could conclude that our slow run gave us a good estimation of our equivalence point and it would help us determine a more accurate pKa value. 2. Based on our quick run data for methoxybenzoic acid, we determined our pKa to be about 5.2. Compared to other students with the same acid, our results are pretty similar.

For this acid, the pKa values of the other lab sections ranged from 5.52 to 5.97. Based on these results, we could conclude we conducted the experiment correctly and determined an accurate pKa for our methoxybenzoic acid. Unfortunately, we did not have enough data points from our slow run data. 3. Based on the lab section data, we concluded the order of increasing acidity of the 6 acids analyzed was p-nitrobenzoic acid > nicotinic acid > p-fluorobenzoic acid > benzoic acid > p-methoxybenzoic acid >p-(N,N-dimethylamino)-benzoic acid. These results are not consistent with the known pKa values of the acids. The order of increasing acidity should be p-nitrobenzoic acid > p-fluorobenzoic acid > benzoic acid > p-methoxybenzoic acid > nicotinic acid > p-(N,N-dimethylamino)-benzoic acid. These results are not consistent with what we know about how certain substituents can affect the acidity of carboxylic acids. This is because p-fluorobenzoic acid contains a fluoride atom, which is electronegative. Electronegative substituents increase the acidity of carboxylic acids by stabilizing their conjugate-base carboxylate ions.

Summary: : The purpose of this experiment was to determine the pKa of a series of para-substituted benzoic acids to understand how the substituents affect the ionization of the acids. To accomplish this, we titrated methoxybenzoic acid to find its equivalence point. We did two different titrations and our first one was a quick titration. During this, we found a general area of where the spike in pH is depending on how much of the titrating base we used. From there, using the range that we found, we did a slow titration in which we added NaOH drop by drop to the methoxybenzoic acid. Using the pH probe to monitor the change in acidity, we could use the data points that we found and put them in an excel sheet. Using both of the first and second derivative graphs, an equivalence point was found. With the graphs, we could find the middle of the slope where methoxybenzoic acid began to rapidly change into its conjugate base. With the equivalence point, we found the pKa to be about 9.5. We had to obtain data from another lab group since our solution was incorrect. This may have been the result of diluting our reaction solution incorrectly, adding NaOH too quickly while completing our titration, or may have been from our pH probe not being calibrated correctly. In the future, this experiment could be improved by being more precise when making our reaction solution, setting up the experiment, and while recording our values during our titration. These fixes will help insure that we experimentally determine a more accurate pKa value for methoxybenzoic acid....